Method of making porous metal tubes



United States Patent 3,313,622 METHOD OF MAKING POROUS METAL TUBES Pierre Potet, Grenoble, France, assignor to Societe des Poudres Metalliques et des Alliages Speciaux Ugine- Carbone, Paris, France, a corporation of France N0 Drawing. Filed Mar. 16, 1964, Ser. No. 352,349

5 Claims. (Cl. 75-222) This application is a continuation-in-part of my copending application Ser. No. 110,877, filed May 18, 1961, now abandoned, and relating to Method of Making Porous Metal Tubes.

This invention relates to a method for production of porous metal tubes, and more particularly to a method of making thin walled, porous metal tubes which have high porosity, high permeability, high mechanical strength, and substantially uniform sized pores. These tubes are made from a metal powder and an extrusion binder which are formed into a paste and then extruded to pro- .duce the tubes. Thereafter, the tubes are sintered.

To obtain the desired physical properties in the tubes, it is necessary to use a rather coarse-grained metallic powder of a spherical or globular form and of uniform diameter (ranging from a few microns to a few hundred microns) and to use a large proportion of the binder.

Forming thin walled, metal tubes in great lengths from metal powder presents difiiculties due especially to lack of cohesion and homogeneity of the product in different intermediate stages of its fabrication, and to risk of deformation resulting therefrom.

My invention overcomes these problems and relates to a method of making porous metal tubes which have high porosity, high permeability, high mechanical strength, and substantially uniform sized pores. Specifically, the method comprises forming a paste of ,a metal powder and an extrusion binder which is volatile at the sintering temperature of the metal powder, by mixing the metal powder and the binder at a temperature at which the binder is liquid. The metal powder is in a tapped density state and the amount of binder used in preparation of the paste is substantially equal to the volume of pores resulting from the metal powder being in the tapped density state. Following preparation of the paste, it is cooled to substantially ambient temperature and then extruded into a tube at substantially ambient temperature. Next, this tube is placed in a bed of inert powder and the interior of the tube is filled with this inert powder, which has a specific surface such that internal pore surface of the grains thereof is substantially about -100 times that of the external surface of the grains. Thereafter, the tube and the inert powder are heated, first slowly so that the inert powder absorbs the binder which is progressively substantially eliminated from the tube as it melts, and then the heating is continued to the sintering temperature of the metal powder and the binder is volatilized from the inert powder and passes off.

Preferably, the inert powder is compacted, especially that inside the tube, so that radial shrinkage of the tube does not occur during sintering.

The initial metallic powder is preferably well gauged, in a granular and as far as possible spherical form, and the dimensions of its grains are between substantially about 1 and 500 microns. In addition, this metal powder used in preparation of the paste is in the tapped density state which is obtained by settling the powder through tapping against its container until the volume of the powder is substantially constant. This tapped density varies considerably according ot the texture and form of the grains of the powder, and may reach 70% of theoretical density when the grains are spherical and have uniform Patented Apr. 11, 1967 diameters. On the other hand, it can be as little as 30% when the grains are porous and irregular in shape.

In the tapped density state, the grains have many surfaces in contact with surfaces of other grains, and porosity is highmore than 50%. Additionally, compression of the powder is not required to obtain this tapped density state. Generally, for a given metal powder, tapped density corresponds to i3% of a given value.

The binder is preferably non-aqueous, and examples of same include paraffin, Vaseline fat, waxes, etc.

Preparation of the paste comprises intimately mixing the metal powder in the taped density state and the binder, for example, mixing by kneading under vacuum. The mixing takes place at a temperature .at which the binder is liquid, for example, 60 C.-70 C., to render the binder easily mixable with the powder. The amount of binder used is that sufiicient to fill the empty spaces between the grains of the metal powder in the tapped density state.

Upon completion of mixing the paste, it is poured into a container of an extrusion press and allowed to solidify and cool to ambient temperature. At ambient temperature, the paste is sufiiciently firm to provide the extruded tube with a mechanical strength so that it avoids selfdistortion, but is plastic enough to be extruded.

After the cooling, the paste is extruded at substantially ambient temperature through a die provided with a mandrel to form a tube. A spindle in axial alignment with the mandrel receives the extruded tube and supports it. The extrusion may be carried out in a vertical direction or at an angle to the vertical.

The pressure employed in extrusion depends upon the quality of the paste and upon the die section. The higher the diiference between the cross-section of the press container and the cross-section of the extruded tube, the higher the extrusion pressure. Inasmuch as the tube is thin walled, the extrusion pressure is in suitable proportion to the cross-section of the container.

Following the extrusion operation, the extruded tube is placed in a bed of a chemically inert powder for the sintering operation. This bed supports the tube during sintering and can be formed in a sintering pot where the tube is disposed vertically after which the pot and the interior of the tube are filled with the inert powder. Sometimes, a support rod of less diameter than the tube is located in the axis of the tube and the intermediate space between the rod and the interior walls of the tube are filled with this inert powder.

The inert powder of the bed has a high specific surface and is composed of materials such as alumina, graphite, magnesium silicate, etc. Preferably, the bed has an apparent density of substantially about 20%-25% of theoretical density, and is a tapped piling of the chemically inert grains of the powder which are fine enough to avoid marking the surface of the extruded tube when the bed is prepared. Additionally, the grains are regularly rounded in shape to insure easy-pouring and compacting and are sufiiciently large to escape plugging the pores of the sintered tube. The grains of the bed are porous so that they absorb melted binder during the sintering operation. In a grain, the volume of the pores is about 30% to 70% of the volume of the grain, and the diameter of the pores is narrow enough to eflect quick absorption of the melted binder by capillary action. For example, the diameter of a grain is from d/ to d/ 10, where d represents the diameter of the grain and is about 20;; to 200 dependent upon the grandulometry of the sintered tube powder. Thus, the total specific surface of the bed is such that internal pore surface of the grains thereof is substantially about 10-100 times that of the external surface of the grains.

The sintering is carried out by a slow increase in temperature, especially at low temperatures, to effect a progressive elimination of the binder from the powder by melting of the binder followed by absorption of the melted binder by the inert bed. Thereafter, the temperature is raised to the sintering temperature of the metal powder and the binder is eliminated from the inert bed by volatilization. The sintering temperature is experimentally determined so that the final tube has sufficient mechanical strength, while retaining a high porosity.

My invention obtains tubes in long lengths (up to several meters), which have high porosity and uniformly sized pores, together with high permeability and high mechanical strength. Such structure and the high permeability render the tubes highly useful for fine filtering. In some embodiments of the invention, I obtain tubes whose walls are as thin as 0.5 mm.

EXAMPLE temperature of 60 C.:

Percent by weight Nickel powder 80 Paraffin 14 Vaseline fat 6 This liquid mixture was poured into the container of I ""an extrusion press of 100 cm. in length and 50 cm. in

diameter, then allowed to cool to room temperature.

The mixture was then col-d extruded at a pressure of 500 kg./cm. through a die inclined at an angle of 45 to the vertical and equipped with a mandrel. Thus, there were obtained, on spindles, 8 tubes of 530 mm. length, 16.1 mm. external diameter, and 14.5 mm. internal diameter.

The tubes were then placed into a bed of alumina grains averaging 30 microns in external diameter, averaging 0.2 microns in internal pore diameter, and having a specific surface of 6 m. /g., contained in a sintering pot. The bed had a relative apparent density of 50% and the external surface of the bed was 0.1 m. /g.

After removal of the spindles, the interiors of the tubes were filled also with alumina and compacted by slight vibrations.

At last, the whole assembly was placed into a pit-furnace under hydrogen atmosphere and the temperature was steadily raised at a rate of 50 C. per hour from room temperature to 850 C. This temperature was maintained during one hour. Afterwards, the assembly was allowed to cool under this hydrogen atmosphere.

Tubes 500 mm. in length, 15.3 mm. in external and 14.3 mm. in internal diameter were thus obtained. These tubes had a porosity of 45%. Their pores (measured with a mercury porosimeter) were of sizes comprised between 4 and 8 microns. Air diffused through them at a rate of 20 liters/hour per cm. of apparent surface at a pressure difference of 1 cm. Hg.

The tubes withstood an internal over-pressure of 15 kg./cm.

While I have describe-d a preferred embodiment of my invention, it may be otherwise embodied within the scope of the appended claims.

I claim:

1. A method of making thin walled, metal tubes of high porosity, high permeability, high mechanical strength and having substantially uniform sized pores which 'comprises,

(A) mixing metal powder and anextrusion binder at a temperature at which said binder'is liquid to form a paste,

(1) said metal .powderbeing in a tapped density state, (2) said binder being volatile at the temperature of the sintering temperature of the powder, and (3) the amount of said binder which is mixed with the metal powder being substantially equal to the volume of pores present in the metal pow der when it is in the tapped density state,

(B) cooling said paste to substantially ambient temperature,

(C) extruding said paste to form a tube at substantially ambient temperature,

(D) placing said tube in a bed of powder inert to the metal powder and the binder under the prevailing conditions, and filling said tube with said inert powder, said inert powder having a specific surface such that the internal pore surface of the grains thereof is substantially about 10 to 100 times that of the external surface of said grains,

(E) heating said tube and said inert powder to melt the binder and transfer the binder from the tube to the inert powder by absorption, and

(F) further heating said tube and said inert powder to sinter the metal powder in the tubeand volatilize the binder in said inert powder,

2. The method of claim 1 characterized by said metal powder being nickel and having a tapped density of 3.55 $0.10.

3. The method of claim 1 ch-aracterizedby said inert powder being alumina.

4. The method of claim 1 characterized by said metal vpowder being nickel and having a tapped density of 3.55 :0. 1'0, and by said inert powder being alumina.

5. The method of claim 1 characterized by said inert powder in said tube being compacted before sintering.

References Cited by the Examiner UNITED STATES PATENTS 2,593,943 4/ 1952 Wainer -222 X 2,792,302 5/1'957 Mott 75222 3,001,871 9/1961 Thien-Chi et al. 75--222 3,052,967 9/1962 Fischer 75-222 3,131,239 4/1964 Calis 75-222 CARL D. QUARFORTH, Primary Examiner. BENJAMIN R. PADGETT, Examiner. A. I. STEINER, Assistant Examiner. 

1. A METHOD OF MAKING TIN WALLED, METAL TUBES OF HIGH POROSITY, HIGH PERMEABILITY, HIGH MECHANICAL STRENGTH AND HAVING SUBSTANTIALLY UNIFORM SIZED PORES WHICH COMPRISES, (A) MIXING METAL POWDER AND AN EXTRUSION BINDER AT A TEMPERATURE AT WHICH SAID BINDER IS LIQUID TO FORM A PASTE, (1) SAID METAL POWDER BEING IN A TAPPED DENSITY STATE, (2) SAID BINDER BEING VOLATILE AT THE TEMPERATURE OF THE SINTERING TEMPERATURE OF THE POWDER, AND (3) THE AMOUNT OF SAID BINDER WHICH IS MIXED WITH THE METAL POWDER BEING SUBSTANTIALLY EQUAL TO THE VOLUME OF PORES PRESENT IN THE METAL POWDER WHEN IT IS IN THE TAPPED DENSITY STATE, (B) COOLING SAID PASTE TO SUBSTANTIALLY AMBIENT TEMPERATURE, (C) EXTRUDING SAID PASTE TO FORM A TUBE AT SUBSTANTIALLY AMBIENT TEMPERATURE, (D) PLACING SAID TUBE IN A BED OF POWDER INERT TO THE METAL POWDER AND THE BINDER UNDER THE PREVAILING CONDITIONS, AND FILLING SAID TUBE WITH SAID INERT POWDER, SAID INERT POWDER HAVING A SPECIFIC SURFACE SUCH THAT THE INTERNAL PORE SURFACE OF THE GRAINS THEREOF IS SUBSTANTIALLY ABOUT 10 TO 100 TIMES THAT OF THE EXTERNAL SURFACE OF SAID GRAINS, (E) HEATING SAID TUBE AND SAID INERT POWDER TO MELT THE BINDER AND TRANSFER THE BINDER FROM THE TUBE TO THE INERT POWDER BY ABSORPTION, AND (F) FURTHER HEATING SAID TUBE AND SAID INERT POWDER TO SINTER THE METAL POWDER IN THE TUBE AND VOLATILIZE THE BINDER IN SAID INERT POWDER. 